Device for playing optical discs

ABSTRACT

A disc drive apparatus ( 1 ) for optical discs ( 2 ) comprises: a frame ( 3 ); a sledge ( 10 ) displaceably mounted with respect to said frame ( 3 ); a lens actuator ( 43, 21 ) displaceably mounted with respect to said sledge ( 10 ); and a control unit ( 90 ) for generating a control signal (S CL ) for the lens actuator ( 43, 21 ). The control unit ( 90 ) is designed, during a jump operation, to generate said control signal (S CL ) for the lens actuator ( 43, 21 ) on the basis of an actuator deviation signal (S AS ) representing a difference between actuator position (X A ) and sledge position (X S ).

The present invention relates in general to a disc drive apparatus forwriting/reading information into/from an optical storage disc;hereinafter, such disc drive apparatus will also be indicated as“optical disc drive”.

As is commonly known, an optical storage disc comprises at least onetrack, either in the form of a continuous spiral or in the form ofmultiple concentric circles, of storage space where information may bestored in the form of a data pattern. Optical discs may be read-onlytype, where information is recorded during manufacture, whichinformation can only be read by a user. The optical storage disc mayalso be a writable type, where information may be stored by a user. Forwriting information in the storage space of the optical storage disc, orfor reading information from the disc, an optical disc drive comprises,on the one hand, rotating means for receiving and rotating an opticaldisc, and on the other hand optical means for generating an opticalbeam, typically a laser beam, and for scanning the storage track withsaid laser beam. Since the technology of optical discs in general, theway in which information can be stored in an optical disc, and the wayin which optical data can be read from an optical disc, is commonlyknown, it is not necessary here to describe this technology in moredetail.

For rotating the optical disc, an optical disc drive typically comprisesa motor, which drives a turntable engaging a central portion of theoptical disc. Usually, the motor is implemented as a spindle motor, andthe motor-driven turntable may be arranged directly on the spindle axleof the motor.

For optically scanning the rotating disc, an optical disc drivecomprises a light beam generator device (typically a laser diode), anobjective lens for focussing the light beam in a focal spot on the disc,and an optical detector for receiving the reflected light reflected fromthe disc and for generating an electrical detector output signal.

During operation, the light beam should remain focussed on the disc. Tothis end, the objective lens is arranged axially displaceable, and theoptical disc drive comprises focal actuator means for controlling theaxial position of the objective lens. Further, the focal spot shouldremain aligned with a track or should be capable of being positionedwith respect to a new track. To this end, at least the objective lens ismounted radially displaceable, and the optical disc drive comprisesradial actuator means for controlling the radial position of theobjective lens.

More particularly, the optical disc drive comprises a sledge which isdisplaceably guided with respect to a disc drive frame, which frame alsocarries the spindle motor for rotating the disc. The travel course ofthe sledge is arranged substantially radially with respect to the disc,and the sledge can be displaced over a range substantially correspondingto the range from inner track radius to outer track radius. Said radialactuator means comprise a controllable sledge drive, for instancecomprising a linear motor, a stepper motor, or a worm gear motor.

The displacement of the sledge is intended for roughly positioning theobjective lens. For fine-tuning the position of the objective lens, theobjective lens is displaceably mounted with respect to said sledge, andthe optical disc drive comprises a lens actuator for displacing theobjective lens with respect to said sledge. The design can be of a typewherein the objective lens follows a substantially straight line ofdisplacement with respect to the sledge, or of a pivoting type, whereinthe objective lens follows a curved line of displacement with respect tothe sledge. The displacement range of the objective lens with respect tothe sledge is relatively small, but the positioning accuracy of theobjective lens with respect to the sledge is larger than the positioningaccuracy of the sledge with respect to the frame.

In the following, the objective lens and the actuator will be consideredas an integral whole, and a displacement of the objective lens withrespect to the sledge will simply be indicated as a displacement of thelens actuator. Further, unless specifically mentioned otherwise, thewording “the actuator” will indicate the lens actuator.

Normally, just following the track of the rotating disc only requiresrelatively low radial velocities. However, sometimes a jump to anothertrack is required. An important characteristic feature of a disc driveis the access time, i.e. the average time needed to reach a desiredlocation on disc. Herein, a great role is played by the time needed tocomplete a jump to the target track. Therefore, jumps are associatedwith relatively large radial velocities and, as a consequence, withrelatively large radial accelerations.

A problem in this respect, associated with large accelerations, is thefact that the actuator has a certain mass inertia. On acceleration ofthe sledge, the actuator tends to stay behind; on deceleration of thesledge, the actuator tends to shoot ahead. Further, the actuator tendsto resonate, especially when the sledge accelerates or comes to astandstill.

In order to mitigate these problems, it is already known to dampen theresonance of the actuator. Further, when a jump is about to be made, itis already known to provide the actuator control with the sledge controlsignal. This is indicated as a feed-forward control.

In the prior art, damping is effected only with reference to the disc.To this end, the photo-detector output signal is analyzed to deriveinformation with respect to track-crossings, and the number of trackscrossed per unit time is monitored. This information is compared withthe sledge control signal, and deviations are counter-acted by theactuator.

This type of control requires a calculating step, wherein the number oftrack-crossings as counted is compared with the number oftrack-crossings as expected. Further, in cases where the track crossingsignal is absent, or contains relatively much noise, it may be difficultto achieve reliable and robust control on the basis of countingtrack-crossings alone. For instance, if the track crossing signalcontains a lot of noise, it may happen that erroneous track crossingsare detected.

An important objective of the present invention is to provide animproved actuator control which allows faster access, especially incases with an unreliable or noisy track crossing signal.

According to one aspect of the present invention, during a jump, theactuator control signal is generated while taking reference to thesledge itself. Then, without the calculations of track-counting, it willbe possible to force the actuator to more accurately follow the movementof the sledge during jumps.

It is of course possible to provide a separate detector for detectingmovement of the actuator with respect to the frame, and to compare theactuator position thus detected with the sledge control signal. However,in a preferred embodiment of the present invention, a measuring signaldirectly proportional to the difference between sledge position andactuator position is derived from the output signal of the opticaldetector. A preferred example of such measuring signal is the DC levelof the one-spot push-pull signal.

Thus, according to an important aspect of the present invention, in adisc drive according to the present invention, an actuator drive systemis adapted to receive the one-spot push-pull signal, to perform alow-pass filter operation on this signal, and to take this low-passfiltered signal into account when generating a control signal for theactuator.

As mentioned, the shape of the tracks is, in practice, not perfectlycircular with respect to the rotational axis of the disc. This may becaused by the fact that the tracks themselves do not have an idealspiral shape or circular shape, or by the fact that the centre hole ofthe disc is not exactly centred, or by the fact that there is some playbetween centre hole and turntable. In a tracking mode, i.e. when theactuator is controlled to follow a track, the actuator will constantlybe moved to stay on track (i.e. the deviation between actuator positionand track position is controlled to always be zero). If the actuatorwould be held perfectly still, the tracks would shift with respect tothe actuator, and the actuator has lost track (i.e. the deviationbetween actuator position and track position varies constantly).

In an access mode, the sledge is controlled to jump to another track,wherein the jump distance is based on the calculated distance betweenpresent track and target track. If the actuator would be rigidly fixedto the sledge, indeed, by the time the sledge arrives with the actuatorat the calculated position of the target track, the actual position ofthe target track has shifted. The result is that after the jump, thesystem finds itself on the wrong track, and a new jump (retry) isnecessary to the target track. This, of course, adds to the access time,and should be avoided as much as possible.

Therefore, in order to take into account possible mechanical aberrationsand eccentricity of the disc, the present invention proposes to providethe actuator with a shape memory, containing information on the shape ofthe disc and/or the tracks. It may prove sufficient if only theeccentricity of the disc is recorded.

In practice, whenever a new disc is introduced into the disc drive, thedisc drive will perform a shape test on the disc or tracks, whereinduring at least one disc revolution the shape of a track is measured,and the results are stored in the memory.

Then, when performing a jump, the actuator control takes into accountthe shape information recorded in said memory, by giving the actuator aposition correction with respect to the sledge position, correspondingto the track shape and/or eccentricity.

These and other aspects, features and advantages of the presentinvention will be further explained by the following description of apreferred embodiment of an optical disk system according to the presentinvention with reference to the drawings, in which same referencenumerals indicate same or similar parts, and in which:

FIG. 1 schematically shows a functional block diagram of an optical diskplayer or recorder;

FIG. 2 schematically illustrates a radial jump;

FIGS. 3A-3C illustrate different methods for generating an actuatordeviation signal;

FIG. 4 is a graph illustrating a one-spot push-pull signal;

FIG. 5 is a block diagram schematically illustrating a first embodimentof a control circuit;

FIG. 6 is a block diagram schematically illustrating a second embodimentof a control circuit;

FIG. 7 is a block diagram schematically illustrating a third embodimentof a control circuit;

FIG. 8 is a block diagram schematically illustrating a fourth embodimentof a control circuit.

Hereinafter, the present invention will specifically be explained forthe case of an optical disc drive for reading information from anoptical disc. However, the present invention is equally applicable to anoptical disc drive for writing information into a recordable opticaldisc.

FIG. 1 schematically illustrates an optical disc drive 1, suitable forstoring information on or reading information from an optical disc 2.The disc drive apparatus 1 comprises an apparatus frame 3. For rotatingthe disc 2, the disc drive apparatus 1 comprises a motor 4 fixed to theframe 3, defining a rotation axis 5. For receiving and holding the disc2, the disc drive apparatus 1 may comprise a turntable 6, which in thecase of a spindle motor 4 is mounted on the spindle 7 of the motor 4.

The disc drive apparatus 1 further comprises a displaceable sledge 10,which is displaceably guided in the radial direction of the disc 2, i.e.in a direction substantially perpendicular to the rotation axis 5, byguiding means not shown for the sake of clarity. A sledge motor,designed for regulating the coarse radial position of the sledge 10 withrespect to the apparatus frame 3, is schematically indicated at 11. Theforce exerted by this sledge motor 11 is schematically indicated asarrows F. Since sledge motors are known per se, while the presentinvention does not relate to the design and functioning of such sledgemotor, it is not necessary here to discuss the design and functioning ofa sledge motor in great detail.

The disc drive apparatus 1 further comprises an optical system 30 forscanning tracks (not shown) of the disc 2 by an optical beam. Morespecifically, the optical system 30 comprises a light beam generatingmeans 31, typically a laser such as a laser diode, which may be mountedwith respect to the apparatus frame 3 or the sledge 10, and which isarranged to generate a light beam 32 a which passes a beam splitter 33and an objective lens 34. The objective lens 34 focuses the light beam32 b on the disc 2. The light beam 32 b reflects from the disc 2(reflected light beam 32 c) and passes the objective lens 34 and thebeam splitter 33 (beam 32 d) to reach an optical detector 35, which maybe mounted with respect to the apparatus frame 3 or the sledge 10. Theoptical detector 35 produces a read signal S_(R).

With respect to the sledge 10, the objective lens 34 is displaceable inthe radial direction of the disc 2. A radial lens actuator, arranged forradially displacing the lens 34 with respect to the sledge 10, isindicated at 21. Since such actuators are known per se, while furtherthe design and operation of such actuator is no subject of the presentinvention, it is not necessary here to discuss the design and operationof such actuator in great detail. It is noted, however, that theactuator 21 constitutes a radial coupling between the lens 34 and thesledge 10, which coupling has characteristics of elasticity, stiffnessand damping, as is shown schematically at 22.

It is noted that the disc drive apparatus 1 also comprises focus servomeans arranged for axially displacing the lens 34 in order to achieveand maintain focusing of the light beam 32 b exactly on the desiredlocation of the disc 2, but such focus servo means are not illustratedin FIG. 1 for sake of clarity.

The disc drive apparatus 1 further comprises a control unit 90 having afirst output 90 a connected to a control input of the disc motor 4,having a second output 90 b coupled to a control input of the sledgemotor 11, and having a third output 90 c coupled to a control input ofthe lens actuator 21. The control unit 90 is designed to generate at itsfirst output 90 a a control signal S_(CM) for the disc motor 4, togenerate at its second control output 90 b a control signal S_(CS) forthe sledge motor 11 in order to control said force F, and to generate atits third control output 90 c a control signal S_(CL) for the lensactuator 21.

In the following, the objective lens 34 and the actuator 21 areconsidered as an integral whole, and a displacement of the objectivelens 34 with respect to the sledge 10 will simply be indicated as adisplacement of the lens actuator 21. Further, unless specificallymentioned otherwise, the wording “the actuator” will indicate the lensactuator 21.

FIG. 2 illustrates a problem encountered during high-speed access, i.e.a jump 41 from a current track TR0 to a target track TR1. The controlunit 90 comprises a setpoint generator 92 calculating an optimumtrajectory for the sledge 10 in terms of position, speed andacceleration, and generating a corresponding motor control signal S_(CS)for the sledge motor 11, which displaces the sledge 10 carrying theactuator 21. A first part of the problem is that the actuator 21, due toits inertia, can not accurately follow the displacement of the sledge10, i.e. during acceleration it will lag and during deceleration it willovershoot, while further the actuator 21 tends to resonate with respectto the sledge 10, as indicated by arrows 42 in part B of FIG. 2, whichillustrates the arrival of sledge 10 at the calculated position. Asecond part of the problem is that, by the time sledge 10 arrives at thecalculated position, target track TR1 may be displaced with respect tothe calculated position, due to for instance eccentricity.

In order to overcome the first part of the problem, the presentinvention proposes to dampen the movement of the actuator 21 withrespect to the sledge 10. To this end, control of the actuator 21 is atleast partly based on an actuator deviation signal S_(AS) whichrepresents the difference between actuator position X_(A) and sledgeposition X_(S). Herein, X_(A) indicates actuator position with respectto the frame 3, and X_(S) indicates sledge position with respect to theframe 3, as illustrated in FIG. 2.

FIGS. 3A-3C illustrate different methods for generating such actuatordeviation signal S_(AS). For instance, as illustrated in FIG. 3A, it ispossible to have a separate detector 43 sensing the position of actuator21 with respect to the frame 3, which detector 43 generates a measuringsignal S_(A) representing the actuator position. Another detector 44senses the position of sledge 10 with respect to the frame 3, andgenerates a measuring signal S_(S) representing the sledge position. Ina subtractor 45, which may be part of the control unit 90, thesemeasuring signals S_(A) and S_(S) are subtracting to provide actuatordeviation signal S_(AS)=S_(A)−S_(S).

It is noted that the control unit 90 may already be provided with such asledge position detector 44, used in generating the sledge motor drivesignal S_(CS).

Alternatively, it is possible to have a separate detector 46 directlysensing the position of actuator 21 with respect to the sledge 10, whichdetector 43 directly generates the actuator deviation signal S_(AS) asoutput signal.

In a preferred embodiment, the present invention avoids the need of anadditional position detector. Surprisingly, it has appeared possible toprocess the optical read signal S_(R) such as to derive a signalcomponent which is proportional to the radial displacement of theactuator 21 with respect to the light beam 32, i.e. the displacement ofthe actuator 21 with respect to the sledge 10. As illustrated in FIG.3C, the control unit 90 has a read signal input 90 d for receiving theread signal S_(R) from the optical detector 35, and comprises a firstprocessing circuit 47 for receiving the optical read signal S_(R) andfor generating an X-error signal S_(XDN) known in the art as XDN signal.The XDN signal can be derived from the signals coming from satellitespots and the central spot in a 3 spot optical system. The XDN signal isgenerated by adding the signals from the satellite spots, multiplyingthe resulting signal with a weigh factor and subsequently adding thisweighted signal to the signal of the central spot. The value of theweigh factor is such that the weighted signal has an amplitudesubstantially equal to the amplitude of the signal coming from thecentral spot. The XDN signal seems to have a good correlation with theradial displacement of the actuator 21 with respect to the sledge 10. Asalso illustrated in FIG. 3C, the control unit 90 comprises a secondprocessing circuit 48 for receiving the optical read signal S_(R) andfor generating an X-error signal S_(PP) known in the art as one-spotpush-pull signal.

It is possible that the control unit 90 only comprises either aprocessing circuit 47 for generating an XDN signal S_(XDN) or aprocessing circuit 48 for generating an one-spot push-pull signalS_(PP). In such case, however, the control unit 90 can only be used indevices where an XDN signal S_(XDN) can be derived from the optical readsignal S_(R), or in devices where an an X-error signal S_(PP) can bederived from the optical read signal S_(R), respectively. In thepreferred embodiment illustrated in FIG. 3C, the control unit 90comprises the first processing circuit 47 as well as the secondprocessing circuit 48, and further comprises a controllable switch 49having two inputs connected to respective outputs of the two processingcircuits 47, 48 for selectively passing one of the two output signalsS_(XDN) or S_(PP) to its output.

The control unit 90 is designed to control the controllable switch 49 onthe basis of, for instance, the type of disc 2. For instance, as will beclear to a person skilled in the art, the XDN signal S_(XDN) is notavailable in case of DVD-ROM or CD-ROM. In those cases, the one-spotpush-pull signal S_(PP) remains available. FIG. 4 is a graphillustrating the shape of the one-spot push-pull signal during a jump.In this graph, the vertical axis represents signal magnitude while thehorizontal axis represents displacement of the actuator 21 with respectto the disc 2. As illustrated, the one-spot push-pull signal is asubstantially sine-shaped signal having a sloping DC level, i.e. it canbe considered as a superposition of a substantially sine-shaped signaland a substantially linear signal. Each period of this sine-shapedsignal corresponds to a track-crossing by the light beam 32 b. Saidsubstantially linear signal is proportional to a displacement of theactuator 21 with respect to the detector 35.

Ideally, the XDN signal S_(XDN) is a linear signal corresponding to thelinear signal component of the one-spot push-pull signal S_(PP)discussed above. In practice, however, the XDN signal S_(XDN) alsocontains a modulation corresponding to track crossings, but lessstrongly than the one-spot push-pull signal S_(PP).

For removing the modulation, the control unit 90 comprises a low-passfilter 50 receiving the output signal from said controllable switch 49,and providing at its output said actuator deviation signal S_(AS).

It is noted that processing circuits for deriving, from the optical readsignal S_(R), the XDN signal S_(XDN) or the one-spot push-pull signalS_(PP) are known per se, and such known processing circuits can be usedin practicing the present invention. Therefore, a more detaileddescription and explanation of the XDN signal S_(XDN) and the one-spotpush-pull signal S_(PP) and of such processing circuits for generatingthese signals will be omitted here.

According to the present invention, the actuator 21 is preferablycontrolled by a PD control circuit, partly generating the actuatorcontrol signal S_(CL) proportional to the difference between actuatorposition X_(A) and sledge position X_(S), i.e. proportional to theactuator deviation signal S_(AS), and partly generating the actuatorcontrol signal S_(CL) proportional to the velocity of the actuator 21with respect to the sledge 10 to dampen the actuator movements. FIG. 5is a block diagram illustrating an embodiment of a PD control circuit 60suitable to implement such control.

The PD control circuit 60 has an input 61 receiving the actuatordeviation signal S_(AS), and an output 69 providing the actuator controlsignal S_(CL). This output 69 may be connected to the output 90 c ofcontrol unit 90. The PD control circuit 60 has a P-branch 62 forgenerating the proportional part of the actuator control signal S_(CL).This P-branch 62 transfers the actuator deviation signal S_(AS) to afirst input of a first adder 64, having its output connected to thecircuit output 69. Preferably, and as shown, a first amplifier 63 isprovided in the P-branch 62 in order to introduce a predetermined gainfactor K1. Preferably, this amplifier 63 is an adjustable amplifier.

The PD control circuit 60 also has a D-branch 65 for generating adifferential part of the actuator control signal S_(CL). The D-branch 65comprises a differentiating circuit 66 differentiating the actuatordeviation signal S_(AS), i.e. providing an output signal proportional tothe velocity of the actuator 21 with respect to the sledge 10, whichoutput signal is transferred to a second input of a first adder 64.Preferably, and as shown, the PD control circuit 60 comprises a secondamplifier 67 between differentiating circuit 66 and first adder 64, inorder to introduce a second predetermined gain factor K2. Preferably,this amplifier 63 also is an adjustable amplifier.

FIG. 6 is a block diagram illustrating a second embodiment of a PDcontrol circuit 160 suitable to implement such control. In this secondembodiment, the P-branch 62 is identical to the P-branch discussed abovewith reference to the first embodiment 60, but the part of the actuatorcontrol signal S_(CL) proportional to the velocity of the actuator 21with respect to the sledge 10 is generated in a different manner.

The PD control circuit 160 has a second input 71 receiving the opticalread signal S_(R), for instance connected to input 90 d of the controlunit 90. A processing device 72 is connected to second input 71, andprocesses the optical read signal S_(R) for generating a signal S_(AD)indicating the displacement of the actuator 21 with respect to tracks ofthe disc 2. This signal S_(AD) is sine-shaped, a zero-crossing of thissine-shape corresponding to a track-crossing. A crossing counter 73counts the zero-crossings of this signal S_(AD), and generates an outputsignal S73 representing the number of zero-crossings per unit time, i.e.proportional to velocity. A low pass filter 74 smoothens this outputsignal S73, and provides a smoothened velocity signal S_(VD) indicatingthe velocity of the actuator 21 with respect to the disc 2.

The PD control circuit 160 has a third input 76 receiving the sledgemotor drive signal S_(CS) from the setpoint generator 92, which is fedforward to a non-inverting input of a subtractor 75. Said smoothenedvelocity signal S_(VD) is supplied to an inverting input of thissubtractor 75. The subtractor 75 provides an output signal S75proportional to the velocity of the sledge 10 and inversely proportionalto the velocity of the actuator 21, which output signal is transferredto the second input of the first adder 64. Preferably, and as shown, thePD control circuit 160 comprises a third amplifier 77 between subtractor75 and first adder 64, in order to introduce a third predetermined gainfactor K3. Preferably, this amplifier 77 also is an adjustableamplifier.

The second embodiment of PD control circuit 160 can be used in caseswhere the optical read signal S_(R) has clear zero-crossingscorresponding to track-crossings, such as for instance in the case ofCD-ROM or DVD-ROM, or in case of a written portion of a CD-RW or DVD-RWor DVD+RW. The first embodiment of PD control circuit 60 can also beused in cases where the optical read signal S_(R) does not have clearzero-crossings, such as for instance when the track-modulation is onlyweak, such as for instance in the case of X-error signal XDN.

It is possible that the control unit 90 only comprises either the firstPD control circuit 60 or the second PD control circuit 160. In thepreferred embodiment illustrated in FIG. 7, however, the control unit 90comprises a PD control circuit 260 which is a combination of both firstand second control circuits 60, 160. A second controllable switch 80 hasa first input connected to the output of second amplifier 67, and asecond input connected to the output of third amplifier 77, whereas itsoutput is connected to an input of the adder 64. The control unit 90 isdesigned to control this second controllable switch 80 to either use thedifferential control of FIG. 5 or the differential control of FIG. 6.Thus, the control unit 90 is capable of operating in one operative modewhere control is based on the D-branch 65 of first control circuit 60,in which case movements of the actuator 21 are dampened with respect tothe sledge 10, or operating in a second operative mode where control isbased on the velocity signal S_(VD) derived from counting trackcrossings, in which case movements of the actuator 21 are dampened withrespect to the disc 2.

As described above with reference to FIG. 2, a second part of theproblem is that the tracks may have shifted from the location they hadat the moment when the jump was initiated (FIG. 2, at A), due to forinstance imperfect shape of the tracks and/or imperfect alignment of thetracks with respect to the rotational axis 5 of the disc 2. In suchcase, it appears that the jump has terminated at the wrong track, a newjump must be calculated (retry).

In order to prevent such retries, the present invention provides afurther improvement. The control unit 90 is provided with a memory 310containing information on the shape of the tracks of the disc 2. Thismemory 310 will also be referred to as shape memory. During “normal”operation, i.e. track-following operation, the actuator 21 is driven tofollow a track, so that the actuator 21 makes such movements withrespect to the sledge 10 as necessary to stay on track. Consequently, inthis mode, the actuator drive signal S_(CL) generated by the controlunit 90 is representative for the shape of the tracks with respect to astationary sledge. The control unit 90 is designed to store into saidshape memory 310 this actuator drive signal S_(CL), or a track shapesignal S_(TF) derived from said actuator drive signal S_(CL), such asfor instance an average over a predetermined number of tracks, forinstance 10. During a jump, the control unit 90 uses the information insaid shape memory 310 when generating said actuator drive signal S_(CL).

FIG. 8 illustrates a preferred embodiment 360 of the PD control circuit,based on the third embodiment 260, and comprising, in addition to thethird embodiment 260, said memory 310, a tracking repetitive controladder 301, and a compensating repetitive control subtractor 302. Thetracking repetitive control adder 301 is arranged between the output ofadder 64 and the output 69 of the PD control circuit 360. The trackingrepetitive control adder 301 has one input receiving the output signalof the adder 64.

The control unit is designed to generate, on the basis of theinformation in said shape memory 310, a tracking repetitive controlsignal S_(TRC). This tracking repetitive control signal S_(TRC) issupplied to another input of the tracking repetitive control adder 301.Consequently, the actuator 21 is not being held firmly stationary withrespect to the sledge 10, but the lens is made to perform a calculatedoscillatory movement with respect to a reference position which is heldfirmly stationary with respect to the sledge position, this oscillatorymovement corresponding to track movement with respect to the sledge.Thus, when the jump has terminated, the probability that the actuator 21has actually arrived at the desired track has increased.

The above-described operation, based on supplying the trackingrepetitive control signal S_(TRC) to the tracking repetitive controladder 301, is adequate in case of the second embodiment of controlcircuit 160, or if, in the third embodiment 260, the control unit 90 isoperating in said second operative mode. However, in case of the firstembodiment of control circuit 60, or if, in the third embodiment 260,the control unit 90 is operating in said first operative mode, thecontrol circuit would be operative to dampen all movements of theactuator 21 with respect to the sledge 10, effectively counter-actingany affect of the tracking repetitive control S_(TRC). In order to avoidthis, the actuator deviation signal S_(AS) as provided at the input 61of control circuit 360 is modified to allow said calculated oscillatorymovement of the actuator 21 with respect to the sledge 10.

To this end, the compensating repetitive control subtractor 302 has anon-inverting input connected to input 61, and the P-branch 62 and theD-branch 65 have their inputs connected to the output of thecompensating repetitive control adder 302. The control unit 90 isdesigned to generate, on the basis of the information in said shapememory 310, a compensating repetitive control signal S_(CRC), which issupplied to an inverting input of the tracking repetitive controlsubtractor 302.

It is noted that the compensating repetitive control signal S_(CRC) isidentical to the tracking repetitive control S_(TRC), except for aproportionality factor.

It should be clear to a person skilled in the art that the presentinvention is not limited to the exemplary embodiments discussed above,but that various variations and modifications are possible within theprotective scope of the invention as defined in the appending claims.

For instance, the present invention can be practiced in hardware as wellas in software.

Further, within the context of the present invention, it is possible toderive an actuator deviation signal S_(AS) from other sources. Forinstance, in a case where the actuator 21 comprises an electromagneticdevice, a displacement of the actuator 21 with respect to the sledge 10will induce a back-EMF in such electromagnetic device; such back-EMF isperfectly suitable to be received by the control unit 90 in order to beused as actuator deviation signal S_(AS). Thus, although the presentinvention has been explained with reference to an exemplary embodimentwherein the actuator deviation signal S_(AS) is derived from the opticalread signal S_(R), it should be clear that such explanation is notintended to restrict the present invention to such embodiment.

1. Disc drive apparatus for optical discs, comprising: a frame; a sledgedisplaceably mounted with respect to said frame; a lens actuatordisplaceably mounted with respect to said sledge; a control unit forgenerating a control signal (SCL) received by the lens actuator; whereinthe control unit is designed, during a jump operation, to continuouslygenerate said control signal (SCL) for the lens actuator at least partlyon the basis of an actuator deviation signal (SAS) representing adifference between actuator position (XA) and sledge position (XS)irrespective of a position of the lens actuator with respect to anoptical disk.
 2. The apparatus according to claim 1, wherein saidcontrol unit comprises a control circuit having an input receiving saidactuator deviation signal (SAS) and having an output providing said lensactuator control signal (SCL); the control circuit comprising aproportional branch generating a control signal contributionproportional to said actuator deviation signal (SAS).
 3. The apparatusaccording to claim 2, wherein said control circuit further comprises: anadder having an output connected to said circuit output; a firstamplifier having an input coupled to said circuit input and having anoutput coupled to an input of said adder.
 4. The apparatus according toclaim 3, wherein said control circuit further comprises: adifferentiating circuit having an input coupled to said circuit input; asecond amplifier having an input coupled to an output of saiddifferentiating circuit and having an output coupled to an input of saidadder.
 5. The apparatus according to claim 4, wherein said controlcircuit further comprises: a second controllable switch having a firstinput coupled to the output of second amplifier, having a second inputcoupled to the output of said subtractor or said third amplifier,respectively, and having an output coupled to an input of said adder. 6.The apparatus according to claim 3, further comprising: an opticaldetector generating an optical read signal (SR); a setpoint generatorgenerating a sledge motor drive signal (SCS); wherein said controlcircuit further comprises: processing means having an input coupled toreceive said read signal (SR), and designed to process the optical readsignal (SR) for generating an actuator displacement signal (SAD)indicating the displacement of the actuator with respect to tracks ofthe disc; a zero-crossings counter having an input coupled to an outputof said processing means, and designed to generate an output signalrepresenting the number of zero-crossings per unit time; a low-passfilter having an input coupled to an output of said zero-crossingscounter; a subtractor having an inverting input coupled to an output ofsaid low-pass filter, having a non-inverting input coupled to receivesaid sledge motor drive signal (SCS), and having an output coupled to aninput of said adder.
 7. The apparatus according to claim 6, wherein saidcontrol circuit further comprises a third amplifier having an inputcoupled to an output of said subtractor and having an output coupled toan input of said adder.
 8. The apparatus according to claim 2, whereinsaid control unit is designed, in a jump mode, to generate its actuatorcontrol signal (SCL) such as to cause an oscillating movement of thelens actuator corresponding to a track shape.
 9. The apparatus accordingto claim 2, wherein said control unit comprises a shape memorycontaining track shape information, and wherein the control unit, in ajump mode, is designed to read track shape information from said shapememory and to generate a tracking repetitive control signal (STRC) onthe basis of the track shape information in said shape memory; whereinsaid control circuit further comprises: a tracking repetitive controladder having an input coupled to an output of said first adder, havinganother input coupled to receive said tracking repetitive control signal(STRC), and having an output coupled to said circuit output.
 10. Theapparatus according to claim 9, wherein the control unit, in a jumpmode, is designed to read track shape information from said shape memoryand to generate a compensating repetitive control signal (SCRC) on thebasis of the track shape information in said shape memory; wherein saidcontrol circuit further comprises: a tracking repetitive controlsubtractor, having a non-inverting input coupled to said circuit input,having an inverting input coupled to receive said compensatingrepetitive control signal (SCRC), and having an output coupled to theinput end of said proportional branch.
 11. The apparatus according toclaim 9, wherein the control unit is designed to write track shapeinformation into said shape memory when the control unit is in a trackfollowing mode.
 12. Method for controlling a lens actuator during ajump, wherein a control signal (SCL) received by said lens actuator iscontinuously generated at least partly on the basis of an actuatordeviation signal (SAS) representing a difference between actuatorposition (XA) and a sledge position (XS) irrespective of a position ofthe lens actuator with respect to an optical disk.